The hearth of such a boiler is equipped to enable it to be warmed up by an oil burner or a gas burner designed to raise the refractory coating and the fluidized bed charge to normal operating temperature. During normal operation, such a burner is stopped and it is then desirable to protect it from the heat radiation and the erosion caused by the charge of solids in the hearth, which charge contains abrasive grains.
Such protection has already been provided by withdrawing the fuel lance through the burner charging hole and by injecting a considerable grain-sweeping air flow through said charging hole in order to prevent solid grains from the hearth penetrating. However the sweeping air flow is too large and this reduces the efficiency of the installation.
Fuel lances have also been withdrawn and isolated from a hearth by closing the flap valve of a peripheral tube (GB-A-956,885) or by closing a trap door disposed in the burner charging hole U.S. Pat. No. 4,781,576. However, such a trap door is bulky, is not completely airtight, and is subject to thermal expansion that may cause it to jam.
The object of the present invention is to provide a warm up burner which is protected from hearth radiation and from erosion by the particles of the fluidized bed, without requiring the injection of a flow of air that would disturb combustion conditions, while using members that are compact, simple in operation, and undisturbed by thermal expansion.